Abstract
Abstract We report palaeomagnetic results from a 26.5 m sequence of early Miocene sediments in NW Nelson, New Zealand. Analysis of the strong, stable characteristic component of natural remanent magnetization has yielded an important record of the part of Chron 5 from ca. 18.5 to 16.5 Ma, including positive identification of the cryptochron or “tiny wiggle” C5Dr-1, and a pole position ([{«mbda _k} = {78.4^ circ }]; [{☎i _p} = {283.0^ circ }]; dp = 2.2°; dm = 2.8°) for NW Nelson that is indistinguishable from the contemporaneous published pole for the Australian Plate. We infer that this portion of NW Nelson has undergone negligible rotation with respect to the main part of the Australian Plate over the past 17.5 Myr.
Highlights
New Zealand has some of the most complete and accessible Cenozoic stratigraphic records in the world
King (2000) has developed a tectonic model of crustal movements over the past 40 Myr, culminating in the present day configuration of the New Zealand land mass. He divides the New Zealand land mass into 5 tectonic blocks: NW Nelson lies on the Northland—Taranaki— western South Island block, which has remained intact with the Australian Plate while the other blocks have assembled on its eastern margin
Samples (f30, f30A) from the top of the calcareous unit and the base of the mudstone respectively, contain forms transitional from Globorotalia incognita to Globorotalia zealandica, but most likely referable to Globorotalia incognita (Fig. 4(B)–(D)) and we suggest this horizon represents the uppermost part of the Globorotalia incognita zone, lower Altonian, consistent with the macrofossil evidence
Summary
New Zealand has some of the most complete and accessible Cenozoic stratigraphic records in the world. Samples (f30, f30A) from the top of the calcareous unit and the base of the mudstone respectively, contain forms transitional from Globorotalia incognita to Globorotalia zealandica, but most likely referable to Globorotalia incognita (Fig. 4(B)–(D)) and we suggest this horizon represents the uppermost part of the Globorotalia incognita zone, lower Altonian, consistent with the macrofossil evidence. The 26.5 m section sampled spans an interval of approximately 2.0 million years, from which we infer an average accumulation rate of about 13 m per million years This is extremely slow compared with other New Zealand mudstone sequences, some of which accumulated 10 or 100 times faster (e.g. between 1 and 2 m per thousand years in Wanganui Basin, Turner et al, 2005). All specimens carry a substantial viscous component of magnetization, which is removed by demagnetization to about 200◦C In all cases this viscous component is very close to the present day geomagnetic field at Tarakohe Quarry: Dec = 21.4◦, I nc = −66.5◦.
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